The system of fringes of the interferogram of the absorption spectrum of one of the gases, which it is desired to detect, exhibits a periodicity .delta..sigma., measured in wave number. The process standardly used for detection of such a gas consists in using an interferometer set on a path difference adapted to the gas to be studied. In general, the interferogram of a luminous flux having passed through a cell containing the gas to be studied, and the interferometer characterized by a path difference .DELTA..sub.c will exhibit a maximum of contrast of the fringes for .DELTA..sub.c =1/.delta..sigma..
French patent FR-A-2,555,747 describes a device making it possible to use such a process and to detect a gas exhibiting a quasi periodic absorption spectrum. This prior device comprises a light source whose beam goes successively through a concentration lens, a gas cell containing the gaseous mixture to be studied, an interference filter suitable for isolating the total of the specific absorption spectral band of the studied gas, a Fabry-Perrot interferometer consisting of two plates that are separated by a distance e and whose opposite faces are plane and parallel and each covered by a thin layer whose reflecting power is slight, at most 0.5 , over a broad spectral region, and a detector delivering a signal whose maximum amplitude is proportional to the concentration of the studied gas contained in the mixture of the cell.
In this invention of the prior art, the Fabry-Perrot interferometer is used as a 2-wave interferometer, i.e., reflection coefficients of the inside faces of the interferometer plates are slight enough for an incident ray I reaching the interferometer to give rise to only two rays T0 and T1 in which almost the total of the energy of the incident ray is found, primary ray T0 being transmitted directly and indirect ray T1 being reflected once.
The absorption region of a gas studied corresponds to a well-defined path difference of the interferometer, and therefore to a distance between the interferometer plates characteristic of the gas.
In the invention of the prior art, it is possible to adapt at best the distance between the plates, to find one that corresponds to the particular gas studied and which leads to a maximum signal, by moving at least one of the plates of the interferometer with the help of piezoelectric ceramics supplied with direct current.
In a preferred embodiment of the prior art, the 2nd plate of the interferometer is mounted on piezoelectric cells supplied with alternating current, so that the distance between the plates varies between e-.delta.e and e+.delta.e at a frequency f linked to that of the current. This operation has the sole purpose of modulating the output signal, the modulation being understood here by the repetition, at frequency f of a portion in the vicinity of its maximum, of the intensity curve as a function of the interplate distance, to allow a more precise reading of this maximum.
In another device of the prior art, the Fabry-Perrot interferometer is replaced by an interferometry and modulation unit consisting of a polarizing film acting as a polarizer, a birefringent plate with thickness e' and birefringence .DELTA.n providing a path difference .DELTA..sub.c =e'.times..DELTA.n, another polarizing film acting as an analyzer, the width of the birefringent plate being selected as a function of its birefringence so that a maximum signal is collected on the detector for the spectrum of the gas to be identified.
These devices, considering existing materials that can be used, are not excessively costly and make it possible to process gases exhibiting a fine absorption structure in the spectral region between 0.1 and 5 microns.
Such devices exhibit a great specificity because it is necessary, for a second gas to provide a signal that absorbs in the same wavelength region defined by the filter and with a periodicity very close to that of the gas for whose study the device is more particularly adapted.